Since my colleague Powder Oracle is providing new updates practically every day in the wonderfully productive pre-Christmas period, I would like to take this opportunity to look at the somewhat more timeless nature of snow. This temperature gradient causes a vapor pressure gradient. Vapor pressure is a quantity that describes the partial pressure of a gas. In our case, this gas is water vapor. Snow molecules move along this vapor pressure gradient from bottom to top, from higher to lower pressure. They detach from snow grains on the ground and grow back on the next higher ones. Molecules then detach from these again and so on. Of course, our molecules do not simply grow back on a snowflake somewhere. They avoid sharp branches, as they occur on snowflakes, and look for cozy hollows where they can nestle. The vapor pressure is not only temperature-dependent, but also dependent on the surface curvature or the interfacial energy of a surface, which in turn means that the vapor pressure is higher over strongly curved surfaces (peaks or branches) than over flatter surfaces (Gibbs-Thomson effect). As a result, all the peaks soon disappear and our snowflakes develop over time into rather angular grains with smooth edges: The dreaded floating snow or deep frost.
It is known to be quite unbinding and forms a snowpack foundation that is extremely susceptible to disruption. This is exactly what can be seen in the profile from Obergurgl. Here, half of the snowpack still consisted of floating snow on 16.12. An ice lamella is embedded in it and a melt crust forms the vapor barrier at a height of just over 40 cm, which acts like a lid for the deep snow. Above this lies the snow of the last 10 days, which has fallen under the influence of some wind. Accordingly, two windblown snow caps can be seen.
The snow cover in the profile from 20.12. is almost entirely from last week's snowfall, only the hard layer at the bottom was probably there before. Floating snow has not (yet) formed. The snow in the lower layers has not yet completed its decomposition process. We can see that the firmness of the snow cover is generally decreasing towards the top, not the other way around as in Obergurgl. At the top, we can see a very light, relatively soft windblown harsh cover and at around 50 cm we can see a small melt crust, which probably originates from a rain event in the low and medium altitudes of the northern Alps on December 16th. Deep frost can form on such crusts, which then makes life difficult for us as a treacherous, embedded weak layer.
Just about every change in the weather, and sometimes even the non-changes, are reflected in the snow cover. On Thursday (22.12.) we can expect a little more fresh snow, on Friday we should expect the one or other cap to form on the sunny side and the snow to generally become heavier. On Christmas Eve, another cold front will bring more fresh snow, which may or may not somehow combine with the layers below. Stable high-pressure weather is on the cards for next week, whereby the snow cover may settle depending on the temperature, or the formation of floating snow will be favored and weak layers will be preserved.
